The inventive concept relates generally to semiconductor memory devices. More particularly, the inventive concept relates to a nonvolatile memory device and a memory system comprising the same.
Semiconductor memory devices can be roughly divided into two categories according to whether they retain stored data when disconnected from power. These categories include volatile semiconductor memory devices, which lose stored data when disconnected from power, and nonvolatile semiconductor memory devices, which retain stored data when disconnected from power. Examples of volatile semiconductor memory devices include dynamic random access memory (DRAM) and static random access memory (SRAM), and examples of nonvolatile semiconductor memory devices include mask read-only memory (MROM), programmable ROM (PROM), erasable programmable ROM (EPROM), and electrically erasable programmable ROM (EEPROM).
Flash memory is a representative example of an EEPROM. Flash memory device is commonly used to store many forms of data (e.g., voice and image data) in information appliances such as computers, cellular phones, personal digital assistants (PDAs), digital cameras, camcorders, voice recorders, MP3 players, handheld PCs, game machines, facsimile machines, scanners, printers, and so on.
In an attempt to provide increased storage capacity, researchers have developed flash memory devices capable of storing multiple bits of data per memory cell. Such devices are commonly referred to as multi-level cell (MLC) or multi-bit memory devices. Because of the way they are designed, MLC memory devices may experience tighter operating margins compared with memory devices storing only one bit of data per memory cell. Consequently, they may be more susceptible to errors, defects, and/or diminished performance.